1. Field of The Invention
The present invention relates generally to a fuel-purging system used for a fuel processing system which processes vaporized fuel in a fuel tank before the fuel is introduced into an internal combustion engine of, for example, an automotive vehicle. More specifically, the invention relates to a self-diagnosing fuel-purging system which can detect abnormality in itself.
2. Description of The Background Art
Japanese Patent Second (examined) Publication (Tokko Sho.) No. 53-19729 discloses a fuel-purging system used for a fuel processing system which processes vaporized fuel in a fuel tank. In this system, when the pressure within the fuel tank becomes a positive pressure greater than a predetermined value, the vaporized fuel in the fuel tank is introduced into a canister packed with an adsorbent, such as activated carbon, to be adsorbed onto the adsorbent. Then, the adsorbed fuel is removed from the adsorbent to be supplied to an engine through a purging passage. The purging passage is provided with a diaphragm valve which is controlled to be open and closed in accordance with negative throttle pressure. This diaphragm valve is suitably controlled so that fuel-purging is performed in a predetermined fuel-purging condition.
In such a fuel processing system, when the purging passage is clogged or if leaks are present, normal fuel-purging can not be performed. In order to assure reliability of the system, it is desirable that the system be self-diagnosing.
In conventional fuel-purging systems, abnormality is determined on the basis of variation of temperature of the adsorbent. The reason for this is that temperature of the adsorbent increases when vaporized fuel is charged or adsorbed thereon, and decreases when the adsorbed fuel is purged therefrom. Therefore, in conventional fuel-purging systems, abnormality is determined by detecting temperature rise of the adsorbent in a fuel-purging prevented condition in which fuel-purging is prevented, and temperature drop of the adsorbent in a fuel-purging condition in which fuel-purging is performed.
However, since switching between the fuel-purging condition and the fuel-purging prevented condition is performed independently of a charging or purging condition of the adsorbent, in the fuel-purging condition the amount of fuel adsorbed before fuel-purging starts to be performed, can not be determined. In addition, since there is little temperature drop when the amount of adsorbed fuel is very small, it is impossible to determine abnormality of the system even if it is determined that there is no temperature drop after fuel-purging starts to be performed.
Accordingly, it is required that abnormality of the system be diagnosed in a condition in which the vaporized gas is adsorbed onto the adsorbent to some degree. In order to accomplish this, an improved process has been proposed. In this process, fuel-purging is stopped for a predetermined period of time before diagnosing, and then, abnormality of the system is diagnosed after determining whether or not vaporized fuel is adsorbed onto the adsorbent on the basis of temperature rise of the adsorbent.
However, if this process is used, since there is little temperature rise of the adsorbent after the fuel is adsorbed onto the adsorbent to some degree, temperature rise can not be detected when the vaporized fuel is adsorbed onto a detected portion of the adsorbent in the initial stage after fuel-purging is performed, or when adsorbing power of the adsorbent is partially lost due to deterioration of adsorbing power of the adsorbent, in an older or used system, for example. In this case, the adsorbing performance can not be determined.